This invention relates generally to numerically controlled machining. In particular, this invention relates to methods for improving accuracy of machining operations conducted under numerical control.
Assuming that there is no lost motion in the machine member drive and that machine member motion is true in the commanded axis, the precision with which machine positioning can be effected by numerically controlled machines is limited by the resolution of the servo mechanisms which effect motion of the machine members. The accuracy with which machining operations can be effected is dependent on these servo mechanisms and other influences such as the deflection, yielding, or bending of machine members relative to the workpiece and thermal deformation of machine members, workpieces, fixtures and tools. The accuracy of the servo mechanisms is relatively constant over time while the other phenomena are dynamic. Therefore, error correction techniques which assume constant errors over the life of the machine are ineffective to correct for errors which are caused by phenomena which vary during normal machine operation. Corrections for such errors may be produced by inspecting a finished workpiece. However, this approach presumes that the inspection of the finished workpiece be accomplished with better accuracy than the accuracy achieved in the machining. This approach has the added disadvantage that the inspected workpiece may be finished with irreparable errors.